Data lost

I bought 500MB data bundle for my Cell C 3G Usb yesterday, I checked after loading the data and there was more than 600MB available. I spent 30 minutes on the internet this morning and received a notification that I have less than 15MB left. I used my usual laptop, didn't do anything unusual on the internet, I didn't download any large files. Where did my data go??? I spoke to someone at the cell C call center and she simply said I need to speak to the technical department to prevent this from happening again. What about the data I lost? This isn't the first time this has happened, but it's by far the biggest loss.